Method for making aspheric surfaces

ABSTRACT

A method of generating aspheric convex surfaces by abrading a spherical surface against a concave cone. The cone being tangent to the spherical surface at a point between the center and edge. The cone tangent against the spherical surface abrades the sphere to produce an aspheric surface having a shorter radius at the center and a longer radius at the edge than the original sphere.

United States Patent Neefe Dec. 18, 1973 [5 METHOD FOR MAKING ASPHERIC 3,064,401 11/1962 Mooney 51/284 SURFACES 3,535,825 10/1970 Volk 51/284 X 3,050,909 8/1962 Rawstron 51/284 x Inventor: Charles N fe, Box g 3,491,489 1/1970 Rudd .1 51/284 x Spring, Tex. 79720 3,471,976 10/1969 Barnett 51/284 [22] Filed: Nov. 1, 1971 pp No: 194,327 Primary ExammerDona1d G. Kelly Related US. Application Data,

Continuation-impart of Ser. No. 15,120, Feb. 27, 1970, Pat. No. 3,641,717.

[52] [1.8. CI. 51/284 [51] Int. Cl 1324b 13/00 [58] Field of Search 51/284, 124 L, 131, 51/216 A, 216 LP [56] References Cited UNITED STATES PATENTS 2,990,664 7/1961 Cepero 51/284 [5 7] ABSTRACT A method of generating aspheric convex surfaces by abrading a spherical surface against a concave cone. The cone being tangent to the spherical surface at a point between the center and edge. The cone tangent against the spherical surface abrades the sphere to produce an aspheric surface having a shorter radius at the center and a longer radius at the edge than th original sphere.

5 Claims, 3 Drawing Figures PAIENIEDHEI: 18 ms 3.778337 SHEEI 2 0f 2 IN VENTOR.

1 METHOD FOR MAKING ASPIIERIC SURFACES This is a continuation-in-part of patent application, Ser. No. 15,120 filed Feb. 27, 1970, by Charles W. Neefe entitled A method and Apparatus for Making Aspheric Convex Surfaces, issued Feb. 15, 1972 US. Pat. No. 3,641,717.

Aspheric surfaces have many desirable uses in all types of optical lenses. Their extensive uses have been limited by the difficulty in producing them in quantity and economically. The object of the present invention is to provide a reliable and economical method of producing large numbers of quality aspheric surfaces. A sphere is a poor refracting surface due to spherical aberration. Spherical aberration is the inability of all parts of a spherical surface to bring parallel light rays to a point focus. The periphery of a spherical refracting surface has a shorter focal length than the central area. This condition worsens rapidly with the increase of the aperture. Spherical aberration increases by the square of the aperture. This rapid increase limits the effective use of many larger aperture lenses. Coma and astigmatism of oblique incidents are two off axis manifestations of spherical aberration. If the paths of parallel light are traced through many zones of a large aperture spherical surface, each zone will be found to have a different focal length. If these rays are plotted, a geometric figure, the caustic, will result having no point focus. A zone may be found within the caustic where the rays come closest together. This zone is called the circle of least confusion, and will be located nearer the lens than the focus of the central rays.

If the periphery of the convex lens surface is gradually flattened in precisely the correct degree and place, the caustic may be eliminated and a point focus will result. It has been found that a spherical surface may be changed into such a surface if the area near the edge is flattened slightly. The point of greatest departure from a sphere will be located seventy-one per cent of the distance from the center to the edge. The degree of flattening at the seventy-one per cent point depends upon the diameter of the lens, the radius of curvature, and the refractive index of the lens material. The following formula may be employed to arrive at the maximum depth of the correction required for crown glass plane-convex lenses: x 0.0123D/R.

x is the departure from a true sphere.

D is the diameter of the lens.

R is the focal aperture.

The correction at any given distance from the center of the lens may be found by the following formula: xy .4/41" (N/N-l) U1 y).

y is the distance from center of lens.

A is the aberration coefficient 1.08 for crown glass).

It is the radius of the lens.

F is the focal length.

N is the refractive index (1.52 for crown glass). The

following graph is a typical example of the above point of pressure.

FIG. 2 shows the abrading cone, in section, in place against the lens.

FIG. 3 shows a sphere before abrading, broken line, and the new aspheric curve, solid line.

A method of making convex aspheric surfaces which is capable of quality and production efficiency employs a cone, 8 FIG. 2. The cone may have straight sides as shown in the drawing or the sides may be concave or convex depending on the radius and size of the lens. Small lenses having a short radius may require concave cone sides and large flat lenses convex cone sides. The cone, 8 FIG. 2, is lined with a soft polishing material, 9 FIG. 2, such as soft leather or plastic foam. The lens, 10 FIG. 2, is rotated and the cone, 8 FIG. 2, is held against the rotating lens. A polishing agent such as a metallic oxide (tin oxide, iron oxide or cerium oxide) is applied and the cone is rocked back and forth across the lens.

A series of several cones having different angles may be used. These different cones will be tangent to the lens at different points. By varying the time each cone is used any desired aspheric peripheral flattening may be obtained.

The radius of the central area will develop a shorter radius than the spherical lens with which we started.

' The edge will develop a longer radius than the spherical Your attention is directed to the in which: FIG. 1 shows an abrading method im ploying a single curve with which we started. 11 FIG. 3, the broken line, shows the spherical curve with which we started; the solid line, 12 FIG. 3 shows the new aspheric curve. The space between the solid and broken lines represents the material removed to make the aspheric curve. To reduce the time required, emery grains or other abrasive material may be used to grind the aspheric surface. This must be followed by a polishing operation.

Various modifications can be made without departing from the spirit of this invention or the scope of the appended claims. The constants set forth in this disclosure are given as examples and are in no way final or binding. In view of the above, it will be seen that the several objects of the invention are achieved and other advantages are obtained. As many changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

. Ldaima l. A method of making an aspheric convex surface from a spherical surface comprising:

abrading said spherical surface with the inner surface of a concave cone while moving said cone in an oscillating manner over said spherical surface,

the inner surfaces of said cone being maintained tangent to said spherical surface during said abrading at points between the center and edges of said spherical convex surface.

2. The method of claim 1 and further comprising:

rotating said spherical convex surface during said abrading.

3. The method of claim 1 and further comprising:

oscillating said cone relative to said spherical surface such that the finished aspheric convex surface has a radius inthe center region which is shorter than the corresponding radius of the spherical surface from which it was made.

4. The method of claim 1 and further comprising:

3,778,937 3 4 oscillating said cone relative to said spherical surface from which it was made.

such that the finished aspheric convex surface has 5. The method of claim 1 wherein said aspheric cona radius in the peripheral area which is longer than vex surface is formed on a plastic contact lens. the corresponding radius of the spherical surfaces 

1. A method of making an aspheric convex surface from a spherical surface comprising: abrading said spherical surface with the inner surface of a concave cone while moving said cone in an oscillating manner over said spherical surface, the inner surfaces of said cone being maintained tangent to said spherical surface during said abrading at points between the center and edges of said spherical convex surface.
 2. The method of claim 1 and further comprising: rotating said spherical convex surface during said abrading.
 3. The method of claim 1 and further comprising: oscillating said cone relative to said spherical surface such that the finished aspheric convex surface has a radius in the center region which is shorter than the corresponding radius of the spherical surface from which it was made.
 4. The method of claim 1 and further comprising: oscillating said cone relative to said spherical surface such that the finished aspheric convex surface has a radius in the peripheral area which is longer than the corresponding radius of the spherical surfaces from which it was made.
 5. The method of claim 1 wherein said aspheric convex surface is formed on a plastic contact lens. 